[krishvishal]

Which issue does this PR address?

Relates to #4508

Rationale

Kafka frameworks create topics with specific configurations and read those configurations back. The gateway currently rejects most of them. As a result, Kafka Connect, Kafka Streams, MirrorMaker 2, Debezium, kafka-ui, Terraform, and the Strimzi Topic Operator can stop at startup or when they create their first topic.

This PR proposes the following answers to the three open questions in #4508.

1. Compaction. The gateway accepts cleanup.policy=compact, creates the topic without an expiry or size cap, and logs a warning. Until Iggy supports compaction (Kafka gateway: log compaction for compacted topics #4521), the gateway does not report compact in DescribeConfigs. Persisting the policy requires a new Iggy topic option with a typed field in all eight SDKs.
2. Replication factor. The gateway stores the requested replication factor in the Iggy replication_factor topic option introduced in feat(cluster)!: add per-topic replication factor #4692.
3. Settings Iggy cannot apply. If a setting has no client-visible effect, the gateway accepts any value Kafka accepts and reports the Kafka default in DescribeConfigs. It returns error 40 for message.timestamp.type=LogAppendTime and non-default values of max.message.bytes, because these settings change what clients receive.

The following gaps keep #4508 open.

• Schema Registry reads cleanup.policy on its second startup and stops unless kafkastore.topic.skip.validation=true is set.
• Apicurio Registry 3 reads back cleanup.policy=delete on every startup and fails to start.
• Retention changes apply to a topic created with compact if the alter request omits cleanup.policy.
• Replication factor 2 requires both replicas to be available for writes because Iggy commits on a majority. Kafka can continue accepting writes with one replica down.
• Metadata lists gateways as partition replicas instead of the Iggy nodes that host each partition.

What changed?

• CreateTopics previously returned error 40 for any configuration entry and error 38 for any replication factor other than 1 or -1. It now stores retention.ms, retention.bytes, segment.bytes, and the replication factor as Iggy topic options. It also validates every Kafka 4.4 topic key using Kafka's validation rules and error messages.
• DescribeConfigs previously returned only retention.ms for topics and error 42 for a BROKER resource. It now returns the Kafka 4.4 topic keys for topics and the keys the Strimzi Topic Operator reads for brokers.
• Three new variables control retention and durability for topics the gateway creates. Their defaults preserve the current behavior.

Two changes affect existing clients.

• AlterConfigs now replaces the full topic configuration, matching Kafka's behavior. An empty list resets the topic configuration to its defaults.
• A retention.bytes value smaller than one segment now returns error 40 because Iggy always retains one sealed segment per partition.

gateways/kafka/docs/CONFIGS.md documents each key, the gateway's response, and its limitations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.03%. Comparing base (d0fc19a) to head (ec2fbeb).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #4747   +/-   ##
=========================================
  Coverage     91.03%   91.03%           
  Complexity     3878     3878           
=========================================
  Files          1561     1561           
  Lines        403726   403723    -3     
  Branches     337601   337598    -3     
=========================================
+ Hits         367515   367533   +18     
+ Misses        28851    28827   -24     
- Partials       7360     7363    +3     

Components	Coverage Δ
Rust Core	92.28% <ø> (+<0.01%)	⬆️
Java SDK	79.37% <ø> (ø)
C# SDK	84.42% <ø> (ø)
Python SDK	86.51% <ø> (ø)
PHP SDK	92.23% <ø> (ø)
Node SDK	96.60% <ø> (ø)
Go SDK	71.70% <ø> (ø)
C++ SDK	86.84% <ø> (ø)
see 24 files with indirect coverage changes

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